1. Field of the Invention
This invention relates to apparatus and methods for using Wavefront Coding to improve contrast imaging of objects which are transparent, reflective or vary in thickness or index of refraction.
2. Description of the Prior Art
Most imaging systems generate image contrast through variations in reflectance or absorption of the object being viewed. Objects that are transparent or reflective but have variations in index of refraction or thickness can be very difficult to image. These types of transparent or reflective objects can be considered xe2x80x9cPhase Objectsxe2x80x9d. Various techniques have been developed over the years to produce high contrast images from essentially transparent objects that have only variations in thickness or index of refraction. These techniques generally modify both the illumination optics and the imaging optics and are different modes of what can be called xe2x80x9cContrast Imagingxe2x80x9d.
There are a number of different Contrast Imaging techniques that have been developed over the years to image Phase Objects. These techniques can be grouped into three classes that are dependent on the type of modification made to the back focal plane of the imaging objective and the type of illumination method used. The simplest Contrast Imaging techniques modify the back focal plane of the imaging objective with an intensity or amplitude mask. Other techniques modify the back focal plane of the objective with phase masks. Still more techniques require the use of polarized illumination and polarization-sensitive beam splitters and shearing devices. In all of these Contrast Imaging techniques, modifications to the illumination system are matched to the modifications of the imaging optics.
Contrast Imaging techniques that require phase modification of the back focal plane of the imaging objectives we call xe2x80x9cPhase Contrastxe2x80x9d techniques. These techniques include traditional Phase Contrast as described by Zernike in 1958 (see Video Microscopy, Inoue and Spring, 1997, Plenum Press, NY), those including variations in amplitude and phase on the back focal plane of the objective (see, for example U.S. Pat. No. 5,969,853), variations incorporating spatial light modulators (see, for example, U.S. Pat. No. 5,751,475), and variations of Phase Contrast imaging requiring multiple images (see, for example, U.S. Pat. No. 5,969,855).
FIG. 1 (Prior Art) is a block diagram of a conventional Phase Contrast imaging system 100, which shows generally how Phase Contrast Imaging techniques are implemented. This figure illustrates imaging a phase object 108 through transmission, but those skilled in the art will appreciate that the elements could just as simply have been arranged to show imaging through reflection.
Illumination source 102 and illumination optics 104 act to produce focussed light upon Phase Object 108. A Phase Object is defined here as an object that is transparent or reflective and has variations in thickness and/or index of refraction. Obviously almost any real life object is, strictly speaking, a Phase Object, but only objects having enough thickness or index of refraction variation to be difficult to image will require special imaging techniques. A Phase Object can be difficult to image because the majority of images typically are formed from variations in the reflectance or absorption of the object.
Objective lens 110 and tube lens 114 act to produce an image 118 upon detector 120. Detector 120 can be film, a CCD detector array, a CMOS detector, etc. The Phase Contrast techniques are implemented by using illumination mask 106 and objective mask 112. Traditional imaging, such as bright field imaging, would result if neither an illumination mask nor an objective mask were used.
FIG. 2 (Prior Art) shows a first embodiment of an illumination mask 106a and objective masks 112a, 112b, and 112c constructed and arranged for Phase Contrast Imaging. Illumination mask 106a consists of an annular region 202 of high transmittance and the remaining regions being low to zero transmittance.
Objectives masks 112a, 112b, and 112c have phase and transmittance variations essentially conjugate to the transmittance variations of the illumination mask 106a. With no specimen, the majority of the light from illumination mask 106a will traverse the annular regions (204, 206, or 208) of the objective masks. In objective mask 112a this annular region 204 contains a phase retarding material with the transmittance of each portion of the mask being 100%. In objective mask 112b the annular 206 region contains a phase retarding material as well as amplitude attenuation material. The remaining regions of objective mask 112b have 100% transmittance. In objective mask 112c the annular region 208 contains amplitude attenuation material but no phase retardation material. The remaining regions 210 of objective mask 112c contain phase retarding material and no amplitude attenuation material.
In operation, the light that travels through illumination annulus 202 that is not significantly diffracted by object 108 (as for example when a phase gradient is not present) traverses the conjugate annular regions 204, 206, or 208 of objective masks 112a, 112b, or 112c respectively. When using objective mask 112a this undeviated light is phase retarded. When using objective mask 112b the undeviated light is phase retarded and attenuated. When using objective mask 112c this light is only attenuated, but not phase retarded. The light that is diffracted or scattered by object 108 passes mainly through regions of the objective masks other than the annulus. In objective mask 112a the diffracted light is neither phase retarded nor attenuated. When combined with the undeviated light, brought into phase through the phase retardance at the annulus 204, constructive interference at the image results and the object appears lighter than the background image. In objective mask 112b the diffracted and undeviated light are also brought into phase due to the phase retardance of annulus 206, but the background image intensity is reduced by the amplitude attenuation of annulus 206. In objective mask 112c the diffractive light and the undeviated light are made to destructively interfere at the image so that the image of the Phase Object appears darker in the image than the background. The background is also reduced by the amplitude attenuation of annulus 208. In each of these variations, Phase Contrast imaging converts phase differences in the Phase Object into intensity differences in the formed images.
FIG. 3 (Prior Art) shows a traditional diagram explaining the operation of Phase Contrast imaging accomplished by a conventional imaging system such as 100, in FIG. 1 (Prior Art). See Video Microscopy, Inoue and Spring, Plenum Press, 1997, NY for other similar diagrams. The illumination mask such as 106 produces essentially a hollow cone of light from the condenser. Light that is not diffracted or scattered from the Phase Object passes through the conjugate regions of the objective mask such as the annulus on objective mask 112a. Light that is diffracted or scattered from the Phase Object does not pass through the phase retarding annulus of the objective mask. The diffracted light has been phase retarded by the Phase Object 108. Zernike showed that many Phase Objects can be modeled as imparting a pi/2 phase delay to the diffracted light. When the undeviated light is also delayed by an equivalent pi/2 phase both the diffracted and undeviated light arrive at the image plane in phase and constructively interfere to produce an image of the Phase Object lighter than the background. By changing the relative phases between the diffractive and undeviated light, as well as the relative intensity of the diffracted and/or undeviated light, the image of the Phase Objects can be lighter or darker than the background, and the background intensity can be raised or lowered.
A mathematical description of Phase Contrast imaging is as follows. Represent the incident light wave by sin(wt), where t denotes time and w denotes radian temporal frequency. Assume that illumination mask 106a and objective mask 112b of FIG. 2 are used. When the incident light does not pass through the specimen, this undeviated light intercepts the objective mask at the phase annulus and is phase retarded and attenuated. This light can be represented as:
So=a sin(wtxe2x88x92xcfx86)
where xcfx86 is the amount of phase retardation at the annulus and a, 0 less than =a less than =1, is the transmittance of the light at the annulus. The intensity of the image formed from this signal can be shown to be proportional to the time average of the square of So. This time average is given as:
 less than So2 greater than =a2/2
When the incident light passes through the specimen, the light is delayed (and diffracted or scattered) and can then be represented as:
S1=sin(wtxe2x88x92xcex4),
where xcex4 is the phase delay due to the Phase Object. The value of this phase delay is dependent on the size of the Phase Object, the local surface slope, and the change in index of refraction. It is this phase delay due to the Phase Object that is transferred to image intensity with Phase Contrast imaging. With the identity sin(axe2x88x92b)=sin(a) cos(b)xe2x88x92cos(a) sin(b), S1 can be written as:
xe2x80x83S1=sin(wt) cos(xcex4)xe2x88x92cos(wt) sin(xcex4)
The first term of the specimen-diffracted light S1 is identical to the undeviated light of So with a weighting related to the amount of phase delay due to the Phase Object. This first term will then be modified at the objective mask 112b by being phase retarded and attenuated. The light after the objective is then described as
S1=a sin(wtxe2x88x92xcfx86) cos(xcex4)xe2x88x92cos(wt) sin(xcex4)
Squaring S1 yields:
S12=a2 cos(d)2 sin(wtxe2x88x92xcfx86)2+sin(xcex4)2 cos(wt)2xe2x88x922a sin(xcex4) cos(xcex4) sin(wtxe2x88x92xcfx86) cos(wt)
With the identity sin(a)cos(b)=[sin(a+b)+sin(axe2x88x92b)]/2 this squared signal can be written as:
S12=a2 cos(xcex4)2 sin(wtxe2x88x92xcfx86)2+sin(xcex4)2 cos(wt)2xe2x88x92a sin(xcex4) cos(xcex4)[sin(2wtxe2x88x92xcfx86)xe2x88x92sin(xcfx86)]
The time average of this squared signal can be shown to be given by:
 less than S12 greater than =[a2 cos(xcex4)2+sin(xcex4)2+2a cos(xcex4) sin(xcex4) sin(xcfx86)]/2
For small phase delay xcex4 due to the Phase Object, we can use the approximations that that cos(xcex4)xcx9c1, sin(xcex4)xcx9cxcex4, and xcex42xcx9c0, and can rewrite this time average as:
 less than S12 greater than xcx9c(a2/2)+xcex4 sin(xcfx86)
For no phase retardation at the annulus of the objective mask, xcfx86=0, the value of the time average reduces to (a2/2). This is the same as that of the time average of So2 representing the light that is not deviated by the specimen. Or, as is well known, Phase Objects cannot be imaged with traditional techniques such as brightfield that do not compensate for phase in the undeviated and diffractive light. With Phase Contrast techniques, such as with xcfx86=pi/2 at the objective mask annulus, Phase Objects can be imaged clearly and distinctly from the background image.
Although Phase Contrast Imaging techniques effectively produce high contrast images of Phase Objects, these techniques do not allow a large depth of field or control of general focus-related aberrations. A large depth of field is important when imaging objects that have a depth that is large in relation to the depth of field of the system or when making a very low cost imaging system.
There is a need to improve Contrast Imaging of Phase Objects by increasing depth of field and controlling focus-related aberrations.
An object of the present invention is to improve Contrast Imaging of Phase Objects by increasing depth of field and controlling focus-related aberrations. This is accomplished by using Contrast Imaging apparatus and methods with Wavefront Coding aspheric optics and post processing to increase depth of field and reduce misfocus effects. Increasing depth of field is important when imaging Phase Objects with large depth. Controlling focus-related aberrations is important when making inexpensive Contrast Imaging systems.
Wavefront Coding can be used in conjunction with Phase Contrast imaging techniques to produce systems that have both a large depth of field and high contrast imaging of Phase Objects. The general Phase Contrast imaging system is modified with a special purpose optical element and image processing of the detected image to form the final image. Unlike the traditional Phase Contrast imaging system, the final image using Wavefront Coding is not directly available at the image plane. Post processing of the detected image is required. The Wavefront Coding optical element can be fabricated as a separate component, can be formed integrally with the objective mask, or can be constructed as an integral component of the imaging objective or tube lens, or any combination of such.
A Wavefront Coding optical element can also be used on the illumination side of the system in order to extended the depth of field of the projected illumination due to the duality of projection and imaging. This projected illumination would be broader than without Wavefront Coding, but the optical density as a function of distance from the object would be less sensitive with Wavefront Coding than without. Without Wavefront Coding on the illumination side of the system, the object can technically be imaged clearly but is not illuminated sufficiently. See xe2x80x9cPrincipal of Equivalence between Scanning and Conventional Optical Imaging Systemsxe2x80x9d, Dorian Kermisch, J. Opt. Soc. Am., Vol. 67, no. 10, pp.1357-1360 (1977).
The main component of importance for the Wavefront Coding imaging optics and digital processing in Phase Contrast systems is the objective mask, as opposed to the illumination mask. Without the objective mask, the imaging side of the system (as opposed to the illumination side) is very similar to a traditional imaging system used for imaging non-Phase Objects. The illumination mask can be considered as only required to alter the illumination light of the given object being imaged. Since many combinations of illumination configuration and object can produce the same transmitted (or reflected) wavefront, we can consider the use of specialized illumination as a means of altering the object""s imaging characteristics when the object itself cannot be altered. The illuminated object is imaged through optics that have been modified to enhance the type of wavefront that the specialized illumination system is providing. Only changes in the objective mask necessitate changes in the optics or processing with Wavefront Coding.